My boyfriend (17) has two girlfriends (16 & 16) i dont know what i should do about it.

TL;DR; my boyfriend is unhappy with another girl and i dont know if i should tell her or not.

my boyfriend (M,17) and me (F,16) have been “together” for about a month now. the only problem is that he has been with this other girl for a year and a half. the major issue is that she is my best friend and i met him through her. i know me and my boyfriend are bad people for this but i really do love him and he says he loves me. i know that sounds bad but I never spoke to him with the intention of dating him because i knew he was in a relationship but it was just the way its ended out being.

we are all part of the same friend group and so see each other every day but, because he is cheating on his other girlfriend with me we cant be public about it. i love him so much but when ever he mentions her or i see them together i feel sick and shut down. like for example he stayed at my house last night and it was very nice but this afternoon he asked me to drop him off at my best friend (his other girlfriends) house. i did it because i knew they were together before i became friends with him. but i threw up from crying when i got home and i am just really dont know what to do about it. him and his other girlfriend are always arguing and he says hes not happy with her so i really dont know what to do.

i know they are not going to seperate but im not sure if i should tell her or just end it with him and move on with my life. i dont want to loose my boyfriend but it is horrible feeling like this and i dont know what to do.